High-frequency multiplex signaling system



1am ESPENSCHIED REQUENCY MULTIPLEX' SIGNALING SYSTEM Oct. 6,1925- Filed Sept. 50. 1919 a Sh ts-Sheet 1 W r INVENTOR. I Lyvemzfiwd ATTORNE Oct. '6, 1 25. 1,556,319

L. ESPENSCHIED HIGH FREQUENCY IULTIPLEX SINALING' SYSTEI I Filed Sept. 30, 1919 {s Sheets-Sheet 2 INVENTOR ATTORNEY Oct. 6

Filed Sept. 30, 1919 s Sheets-Shea 5 ATTORNE Patented Oct. 6, 1925. I

UNITED STATES assent PATENT OFFICE.

LLOYD ESPENSCHIED, OE HOLLIS, NEW YORK, ASSIGNOR T0 AMERICAN TELEPHONE AND TELEGRAPH COM PANY, A CORPORATION 0F NEW YORK.

HIGH-FREQUENCY MULTIPLEX SIGNALING SYSTEM.

Application filed September 80, 1919. Serial No. 327,501.

. 1 0 all whomit mag concern:

Be it known that I, LLOYD Esrnnsonmo, residing at Hollis, in the county of Queens and State of New York, have invented certain Improvements in High-Frequency Multiplex Signaling Systems, of which the following is a specification.

This invention relates to high frequency multiplex signaling systems and more par-' ticularly to a terminal circuit arrangement embodyin the use .ofamplifiers adapted to amplify t e side band and the carrier frequency oscillations received by the said terminal circuit."

The broad object of the invention is to' improvethe transmission efliciency and the transmission quality of carrier current circuits. One feature is the provision of a carrier current amplifier for improving the transmission efliciency of the carrier circuit. A further object of the invention is to provide means whereby'the ratio of the unmodulated carrier current transmitted over the system to' the modulated or side band component may be controlledffor the purpose of insuring good quality inthe carrier transmission circuit.

A comprehensive undestanding of the invention will be facilitatedby recognizing that in the ordinary form of carrier current transmission represented by radio, there are ,invo1ved really two channels, one the unmodulated carrier current which is transmitted over the system at the original carrier frequency andthe other the modulated or side band component which is transmitted at a frequency difieringfrom the original carrier frequency by an amount corresponding to the signaling frequency. 'These two channels are commonly treated in radio, for example, as a single channeh'and at the receiving end are fed unseparated into the de i from. the following description when read The 'detecting action obtains, howby virtue of modulation between, that is,- t e beating together of, the currents of the two channels, the carrier and one or both side bands. 'The present invention is concerned with carrier current systems, both wire and wireless, wherein the carrier is transmitted, directly along with the side bands as per ordinary radio practice, as distinguished from the type of carrier system first em loyed for -multiplex service over wires wierein the carrier channel is suptector. ever, only pressed at the sending end-and resupplied at the receiving end by a control, system.

One feature of the invention resides in the use of an amplifier in the high frequency part of the circuit which amplifies simultaneously both the carrier and the side band currents. A given amplification inserted in this manner in the high frequency circuit results in a twofold amplification of the detected current by virtue of the fact that the amplitude of the detected current is a function of the product of the amplitudes of the carrier and the side band currents.

It has been found that for the obtaining of good quality in carrier telephone transmission, it is necessary that the carrier sup ply current be large compared with the side band currents. This relation is necessary in order that in detection or demodulation, the product term of carrier and side band, representing the desired detected current of signaling frequency, be large in amplitude compared with the other product terms such as those resulting from modulation between thetwoside bands or between the component currents of one side band, which currents represent undesired-detected currents tending to interfere with and depreciate the qualityof the transmission. A further feature of the invention is the provision of means whereby the carrier andthe side bands may he amplified in different proportions, that is, the ratio ofthe amplitudes of the carrier to the side hand currents may be variedfor the purpose of improving quality in the manner indicated.

This is'accomplished by combining with the high frequency amplifying apparatus, selective circuits which discriminate as between the carrier frequency and the side band frequency.

This invention will he better understood in connection with the attached drawing, of

which Figure 1 shows an arrangement for' which the carrier current and the side band,

oscillations may be amplified separately, to i the desired degree.

In Fig. 1, L shows a source of low frequency slgnaling currents which is connected with the terminal circuit by means of a transformer T having a network N, 41550- ciated therewithadapted to balance the line L This terminal circuit, which is of the four-wire type, has associated with it low frequency filters F and F and the high frequency filters F and F The transmitting path of this circuit is connected with the branch transmission circuit TL, which is in turn connected with, the'main transmission line ML by means of transformer T A networlrN associated with transformer T is adapted to balance the line ML.

Another circuit L is connected with the main transmission line ML through the low frequency filter F the function of the said line being to transmit low frequency voice currents over the lirle. ML simultaneous with the transmission of the high frequency signaling currents from the terminal circuits. High frequency oscillations received overthe line ML pass through the high fre uency filter F and are impressed on the ranch receivingcircuit RL by the transformer T .Connected be tween the two sides of the four wire circuit is a translating circuit which forms a common path for the outgoing and incoming currents of the terminal circuit The input side of this translating circuit is connected with one side of the terminal circuit by means of the transformer T which is of a type designed for the efiicient transfer of both low frequency and high frequency currents from the terminal circuit to the translating circuit. Connected with the translating circuit is an emplifier A adapted to amplify the currents impressed upon the translating circuit by the transformer T and the currents thus amplified are impressed upon a modulating and demodulating device MD by means of a transformer T The modulating and dea modulating device has associated with it a source of high frequency carrier supply cur- "rent G. The currents resulting from. modulation and demodulation are impressed upon the output side of the terminal circuit by means of the transformer T In the operation of this circuit low frequency signaling currents arising in the circuit/L are impressed by the transformer T upon the low frequency circuit 1 and passing through the filter F are impressed .uponthe translating circuit by the transformer T These low fre uency currents are amplified by the ampli 1cr A and by means of the device MD serve (o-modulate the carrier current. The modulated high frequency oscillat ons are impressed upon the high frequency output circuit from whence they pass-through the high frequency filter F to the transmission I lines TL and Mil Simultaneous with the .transmission of the modulated high frequency oscillations unmodulated carrier oscillations produced by the soulrce G' are l'ansmitted through device MR and over Lunatic the lines TL and ML to the corresponding terminhl circuit at the opposite end of the said line.

The modulated high frequency oscillations received from the terminal circuit at the said other end of the line ML are impressed upon the branch receiving line RL and pass through the filter F a into'the translating circuit. ception of these modulated oscillations unniodulated oscillations of the same frequency are received by the line RL and are impressed upon the translatin circuit passing through the filter F whic 1' is adapted to transmit the carrier frequency and the band of frequencies based thereon. The received band of oscillations and the carrier oscillations of the same frequency are amplified together by the amplifier A and are impressed by means of the transformer T upon Simultaneous with the re the device MD in which they are modulated. The low frequency signaling current resulting therefrom passes throu h the filter F to the line L; and actuates t e low fre q'uency signalin apparatus associated there with. The amp ification of'the oscillations of the side band fr uencies and ofthe ,zar-

rier frequency itsel results'in a two fold amplification of the detected current'by virtue of the fact that the amplitude of delected current is a-function of the roduct of the amplitude of the carrier osc ations and the amplitude of the side band oscillations. Thus an amplification which doubles the received high frequency currents of both the side band and of'the -.carrier supply lated carrier oscillations will pass through a filter similar to F5 and will be impressed upon a translating circuit similar tothat shown in Fig. 1. Although the high fre qucucy amplifier is illustrated in.Fig. 1 applied in a type of circuit wherein one amplifior is common to both sending and receiving it will be understood that the amplifier may be employed in a circuit of any type 'suc as the 22 type circuit wherein separate-hi h frequencies amplifiers are employed 15%! sending and receivin Fig. 2 shows a method by'which theunmodulated carrier oscillations may be separated from the modulated high frequency.

oscillations at the terminal circuit and each set of oscillations may be amplified independently of the other set-of oscillations received at the said terminal circuit. In the figure ll jrepresents the-line over which low frequency current mafy be received and L a line over which low quency current may 'b'e'transmitted. ML represents the main transmiss on-line over which the high frequency oscillations may be transmitted between terminals at opposite ends thereo fi. The line-ML is connected with the input c rcuit l and the output circuit 2 by means of the triple ,winding transformerT having a networkll associated therewith to balance the line ML. The receiving circuit 1 has associated with it an antiresonant circuit 3, v

adapted to ,'prevent oscillations of carrier frequency fom being impressed by the transformer T upon the amplifying circuit A 'Oscillations of carrier frequency are impressed by means of transformerl upon the circuit l, from which oscillations other thanthose of the carrier frequency are excluded by means of the filter F The amplifier A amplifies yscillations of carrier frequency and the amplified output isimpressed acrossz-a resistance or inductance 5 of the duplex translating circuit embracing the demodulating device D The output of the amplifying circuit A is impressed by transformer T across the demodulating circuit, wherein the modulated high frequency oscillations are demodulated by the carrier frequency oscillations, and the low frequency signaling currents resulting therefrom are impressed upon the low frequenc circuit L by the transformer T A low frequency signaling device such as a transmitter is connected with the low frequency line'L which in turn is connected to the input side of the modulator M A source of high frequency carrier oscillations G is adapted to impress said oscillations upon the input sides of an amplifier A and the modulator M The output sides of the amplifier A and the modulator M are connected through selecting circuits 6 and F respectively with transmitting circuit 2, and

the modulated and unmodulated carrier oscillations .are impressed upon the transmission circuit ML by ineans of the transformer T The opposite end of the transmission line ML is preferably equipped with a circuit arrangement for transmitting and receiving, similar to that shown in Fig. 2.

frequencies passing through the transformer T may be represented by n, n-i-f, and nf. By means of a high frequency band filter F the frequencies represented by n+f may be suppressed thereby allowing'the frequencies represented by n and nf to be transmitted to the output circuit 2. Simul taneously the transformer T is transmitting unmodulated oscillations of frequency n, the ainplitude'of which is gi'eatly increased by the amplifier A All of these :oscillations are transmitted over the line ML to the terminal circuit at the other end similar to that shown in Fig. 2. The operation of the circuit at that end of the line will be best understoodby considering what takes place in the" receiving circuit shown in Fig. 2 when similar sets of high frequency oscillations are received over the line ML and impressed upon the circuit 1 by means of the transformer T The received frequencies n and nf will be directed into separate channels by means of suitable devices such as filters or their equivalents. The antiresonant de- --vice 3, which is anti-resonant to the frequency n, prevents oscillations of that frequency from being impressed by the transformer T acrossthe in ut side of the am- 'plifying circuit A but the oscillations of the side band represented by m.f will pass through the said anti-resonant device and will be impressed across the input side of A This side band of oscillations will be amplified and will be impressed by the transformer T across the input side of the detector or demodulator D The carrier freuency n will be impressed by the trans: oriner T, across the circuit 4, from which the oscillations of the side band frequencies n-f are excluded by the filter F The carrier frequency oscillations are impressed by transformer T upon the amplifier A by means of which the carrier frequency may be given the desired degree of amplification. The amplified carrier currents are impressed across the potentiometer 5 in .the common conductor of the input side of the duplex translating device D serving to demodulate the frequencies of the side band n/. The low frequency currents resulting from demodulation are impressed upon the line L by the transformer T It will be seen that the arrangement in Fig. 2 shows means for amplifying the oscillations of carrier frequency separate and distinct from the means for amplifying the oscillations of the side band of frequencies. Thus the carrier frequency may be amplified before transmission by the amplifier A and after reception by the amplifier A. Also i the frequencies of the side band may be a nplified'to a certain degree in the process of modulation by the modulator M1 and the desired band of frequencies may be reainplified separate from the carrier frequency tions of carrier frequency a at the receiving end of the system by means of the duplex or balanced detector circuit as'indicated atll i Fig. 3 shows another terminal circuit arrangement t'or the selective amplification of the carrier frequency and the desired side band frequencies. In the drawing L represents one low frequency signaling cir cuit and L another low frequency signalingcircuit for the reception of the low frequency currents resulting from demodulation of the carrier frequency. L, and L; represent low frequency transmission circuits which are adapted to impress low frequencysignaling currents upon the high frequency transmission system. (i and (i represent two sources of high frequency carrier currents, the frequencies of which may be. represented by n and n respectively. The lines L, and I are connected will the input side of the modulating devices ill. and M respectively, the output sides of which are connected by means of the transformers and T with the output circuit 2, which in turn is connected tothe maintransmission line ML by means of the transformer T A network N adapted to balance the line ML is connected with transformer T Filters F and F associated with the output circuit 2 are adapted to permit the transu'iittal' of oscillations of one of the hands. for example that rernescnted by uf and also oscillations of the carrier frequency I! therethrough but. to suppress o cillations of the frequency n-l-j. These frequencies will. be transmitted through the line ML toa circuit connected with the other end of the said line preferablysimilar to that shown in Fig. 3. The receiving circuit 1 shows two branch receiving circuits connected therewith, which form the receiving circuits for carrier frequencies a and n, and the s de hands a) and ai -f, respectively. The upper receiving circuit, connected withthe low frequency line L has connected therewith a demodulating device 1).. an amplifier A for amplifying the desired side band, and an amplifier A for amplifying the carrier frequency. In a similar manner the other receiving circuit has connected therewith a demodulating device 1) an amplifier A to amplify the side band oscillations,.

and an amplifier A to amplify the oscilla- The method of operation of this circuit is as follows: Low frequency signalling currents repre sented by 7 will be impressed across the input: circuit of the modulator M thereby serving to modulate the high frequency 0scillations a from the source G. Oscillations of the frequencies a, a-l-f, and will be found in the output side of the modulator M Since the filter F is designed to transmit only the carrier frequency /1 and the side band n-y". these frequencies only will be impressed by the transformer T upon the line ML and will be transmitted to the'terminal circuit at; the opposite end of this line. In like manner lowfrequency signaling currents f arising within the line L will be impressed across the input side of the modulator M upon which is also impressed carrier frequency il from the source G. The carrier current will be modulated by the low frequency signaling current and the transformer T will impress upon the output circuit two frequencies repr sei'ited by M n a -H and /i,f Since the filter F transmits only the carrier frequency and the oscillations represented by 71 these "frequencies will be impressed upon the line ML by the transformer T In a similar manner similar sets of frequencies will be transmitted from the circuit at the other end of the line ML and will be impressed upon thereceiving circuit l by the transfm'mer T if we assume that thc'iilters F and F are adapted to pass the carrier frequencies a and a respectively, these frequencies will be impressed by the transformer-s1 4 and T upon the input sides of the amplifiers A and A re spectively. Since the filters F and F are adaptedto transmit the desired side bands, namely those represented by thefrequencies, uf and a f, oscillations of these frequencies will be impressed by transformer 'l and T upon the input side'of the amplifiers A andA. respectively. The amplified oscillations of the carrier and of the side band frequencies in each receiving circuit will'be impressed upon the demodulatingdevices Ii and D and the low frequency signaling currents resulting therefrom will pass to the low frequency lines L and L respectively. This circuit arrangement shows means for transn'iitting a plurality of unmodulated carrierfrequencies, also a plurality of bands of oscillations based upon these carrier frequeruries and for separating each carrier frequency from its accompanying side band in the .receiving circuit 80' that the carrier frequency and the side band frequencies may be separately amplified, and

the amplified oscillations may be impressed upon a demodulating device for detectingthe transmitted signal.

Fig. 4: shows the application of the invention to a terminal circuit of the typefiiha'racterized by a single translating circuit.

adapted to modulate and demodulate the outgoing and incoming currents. In the figure L represents the high frequency line over which it'may be assumed that currents of the frequency a and n-f are being re- 'minal circuit and passing through the low ceived. These frequencies are impressed by the transformer T, upon the input circuit 1. The high frequency band filter l adapted to pass oscillations represented by the frequency nf is inserted in the input side of the terminal which is connected with the translating circuit by the -.transforn1cr T Bridgcdacross the input circuit l is a bypass 2, having therein a filter F adapted to transmit only the carrier frequency 01. The oscillations of this frequency areamplilied by the amplifier A, and are impressed upon the common conductor of the duplex modulating and demodulating device MD,. Associated with the high frequency output branch 3 is a high frequency band filter F F and F represent low frequency filters which are adapted to transmit currents of the signaling frequency, The mode of op eration of this circuit is as follows: Let it be assumed that the carrier frequency a and a band of frequencies represented by nf, are being received over the line L, from the circuit at the opposite end of the said line, and are being impressed upon the receiving branch 1 of the terminal circuit shown in Fig. 4. The band of frequencies represented by nf will pass through the filter F and will be impressed by the transformer '1, upon the modulating and demodulating device MD,. The carrier frequency n will be selectively amplified by the amplifier A, and the amplified oscillations of this frequency will be impressed upon the translating device MD,. The low fre quenoy signaling current resulting from demodulation will be impressed by the transformer T upon the output side of the terfrequencyfilter Fv will be impressed upon the low frequency signaling circuit L The low frequency signaling currents set up in the circuit L by the low frequency signaling apparatus associated therewith will be impressed upon the input branch 4 of the transmitting path and passing through the low frequency filter F, will be impressed upon the translating circuit by the transformer T These low frequency currents will modulate the amplified high frequency carrier current impressed upon the translating circuit by the amplifier A, and the modulated high frequency current resulting therefrom will be impressed upon the output circuit by the transformer T Thesemodulated high frequency oscillations will pass through the high frequency band filter F and over the transmission line L,. It will be seen therefore that by means of the arrangement shown in Fig. 4 the unmodulated carrier oscillations which. are simultaneously transmitted together with one or .more of the side bands based upon this carrier frequency may be separated from the said band or bands in the receiving branch of the terminal circuit and maybe amplified separately from the side bands and impressed upon a translating circuit, common to the transmitting and receiving paths of the terminal circuits.

It will be apparent that this invention provides means whereby the received carrier frequency ,may be amplified simultaneous with the/ amplification of the frequencies impressed in the received side band, the various frequencies being amplified to the same degree, as-in Fig. 1, or the carrier frequency may be separated from the received band of frequencies and each separately amplified, by means of separate selecting circuits as in Figs. 2, 3, and 4:. Although these figures show the invention applied to a carrier system over Wire transmission lines, it is to be understood that it is equally applicable to radio transmission systems. 1 p

Although this invention has been shown as embodied in certaln specific forms and arrangements of parts, it is to be understood that it may be embodied in other and different forms and arrangements than those shown without departing from the spirit and intent of the appended claims.

What is claimed is: 1. In' signaling system the combination of a terminal circuit adapted to transmit and receive oscillations of the side band frequencies and oscillations ofthe carrier frequency, means for separating the side band oscillations and the carrier oscillations, means for amplifying separately each set of oscillations, and means for impressing both sets'of oscillations upon a translating device for detecting the same. j

2. In a high frequency signaling system the combination of a receiving circuit adapted for the reception of oscillations of the side band frequencies and oscillations of the carrier frequency, a detector, means for selecting oscillations of the side band frequencies, separate means for selecting oscillations of the carrier frequency, means individual to each selecting means for amplifying each set of oscillations, and means for-impressing said amplified oscillations upon said detector. 33. In a signaling system the combination of a terminal circuit having a detector associated therewith, and adapted for the'transmission of oscillations of the side band frequenc'ies, and for the reception of oscillations of both the side band frequencies and and unmodulated oscillations, the method for improving the quality of detection of re;

ceived signals, which consists in separating by filtering the modulated oscillations and the unmodulated oscillations, amplifying one of said types of oscillations and combin ing both types of oscillations to produce detection.

' both types of oscillations and combining them to produce detection.

6. In a signaling system characterized by the simultaneous reception of unmodulated carrier frequency oscillations andsideband of oscillations, the method for imp'roving the quality of detection which ,consists in separating by filtering the'carrier frequency oscillations and the oscillationscof the side band frequencies, separately amplifying the two groups of oscillations and simultaneously combining the amplified oscillations to produce detection. a

7. In a signaling system characterized by the simultaneous reception of unmodulated carrier frequency oscillations and side band of oscillations, the method for improving the quality of detection which: consists in separating by filtering the carrier frequency oscillations i'rom the side band oscillations, separately amplifying each group of oscillations, simultaneously combining both groups to'produce detection, and transmitting the resultant detected current while suppressing the carrier frequency current.

8. In a signaling system the combination with a transmission line of a. terminal circuit having transmitting and receiving branches, the receiving branch having means for the separate selection and the separate amplification of the carrier frequency and a side band. of frequencies, and also having a connnon'detection circuit and means for im- Intestimony whereof, I have signed my name to this specification this 29th day- 01":

September, 1919.

' 1 LLGYD ESPENSCHIED. 

